This invention relates to an encapsulation system for semiconductor chips where selected regions on the chip are exposed through the encapsulant.
For many uses of semiconductor devices, it is desired that the devices be encapsulated in some type of inert, protective coating both for protecting the device from the harmful effects of the environment in which the device may be used, which effects may lead to device degradation and failure, and for reducing the possibility that the device will contaminate the environment in which it is used. In a particular class of semiconductor devices sometimes referred to as chemical sensitive field-effect transistor transducers, it is necessary that certain regions on the devices remain exposed through windows or openings in encapsulant material. These devices, several embodiments of which are described in Johnson et al, U.S. Pat. No. 4,020,830, issued May 3, 1977, generally include a semiconductor substrate material, source and drain regions separated by a so-called gate region located at the surface of the substrate material, electrical insulator material overlying the source, drain and gate regions, and a chemical selective system overlying the insulator material above the gate region. The chemical selective system, which generally takes the form of a membrane, is adapted to interact with certain substances to which it is exposed and thereby modulate an electric field produced in the gate region. This modulation is dependent upon the chemical properties of the substances and thereby provides a measure of those chemical properties. Measurements which may be performed include measurement of ion activity, immunochemical concentrations, reducible gas concentrations, and concentrations of enzymes and substrates.
Because chemical sensitive devices are utilized specifically by exposing the devices to various substances and chemical solutions, it is necessary to cover (encapsulate) the entire surface area of the devices except for the chemical sensitive gate regions. Techniques heretofore used for encapsulating chemical sensitive devices include the simple hand dispensation of epoxy or other encapsulant on the device, leaving a window or opening over the gate region. The obvious problems with this technique are that it is time consuming and therefore costly, and difficult to maintain uniformity from one device to the next. Of course, the smaller the device, the more difficult it is to apply an encapsulant by hand and still leave an appropriate window over the gate region.
Another technique which has been considered for applying encapsulant to chemical sensitive devices involves deposition by hand, spinning, or other application technique of epoxy or other encapsulant over the entire surface area of a chip or wafer containing a plurality of devices. The encapsulant is then overlayed with a photoresist material, the material exposed to a predetermined light pattern, and the photoresist material developed. The encapsulant material is etched according to the exposure pattern to create windows over the gate regions of the devices in the chip or wafer. The problem with this technique is that the windows produced do not have vertical walls since the etchant tends to etch in the encapsulant material windows having generally V-shaped cross sections. It is desired that windows formed in encapsulant material have vertical walls to allow close packing of devices on a chip or wafer.